ABSTRACT Rheumatoid arthritis results from a dysregulation in host immunity. Since our initial cloning of murine macrophage migration inhibitory factor (MIF), we have focused on the role of this mediator in the pathogenesis of rheumatoid arthritis. We showed that anti-MIF is protective in experimental arthritis and we reported on MIF's ability to override glucocorticoid immunosuppression, induce sustained activation of ERK1/2 MAP kinases, and promote inflammation by inhibiting activation-induced apoptosis. The importance of MIF in human disease was validated by our discovery of high-expression MIF alleles and their association with rheumatoid arthritis severity; these data were confirmed recently in a larger multi- center study and extended to juvenile idiopathic arthritis. In the currently funded period, we elucidated MIF function by creating genetic models for the complete and cell-lineage specific deletion of MIF (MIF-KO, mif flox), and we developed a knock-in mouse encoding a mutant MIF that resolved questions about the protein's enigmatic tautomerase activity. We also established that MIF signal transduction requires a complex of two proteins: CD74, which binds MIF, and CD44, which initiates signaling by activating Src family tyrosine kinases. In this competitive renewal, we will focus on the function of the recently elucidated MIF receptor. Our Specific Aims are: 1. Define the Role of the MIF Receptor Complex (CD74/CD44) in the Adaptive Immune Response. Our working hypothesis is that signaling through the MIF receptor sustains the survival of activated T lymphocytes and regulates the adaptive immune response. 2. Define the Functional Importance of the MIF Signaling Receptor, CD44, and of CD44 Variant Isoforms in the Immunopathology of Rheumatoid Arthritis. The expression of CD44 splice variants in T cells is associated with an enhanced trafficking response; and in stromal cells, with an increase in invasive phenotype. Our working hypothesis is that MIF induces the alternative splicing of CD44, which regulates MIF-dependent responses and contributes to the immunopathogenesis of rheumatoid arthritis. 3. Identify High-potency, Small Molecule Antagonists of the MIF Binding Receptor (CD74). Structure-function studies indicate that MIF's N-terminal region binds to the CD74 component of the MIF receptor. We will evaluate small molecules that were rationally designed to bind to the MIF N-terminal region for their ability to inhibit MIF interaction with CD74 and ameliorate experimentally-induced arthritis. The information to be gained will have broad application because it will provide mechanistic insight into how the MIF receptor complex (CD74/CD44) contributes to the immunopathology of rheumatoid arthritis. The translational impact of these studies is that a more precise definition of MIF-receptor action will accelerate the development of small molecule MIF inhibitors. Such inhibitors may show greatest efficacy in those patients with high-expression MIF alleles and who manifest an MIF-dependent form of inflammatory disease.